Glucocorticoids (cortisol in humans, corticosterone in mice and rats) are an important class of adrenocorticosteroids that regulate many metabolic and homeostatic processes and form a key component of the response to stress. Glucocorticoids act via intracellular glucocorticoid receptors and, in some tissues, mineralocorticoid receptors; both being nuclear transcription factors. Glucocorticoid action on target tissues depends not only on circulating steroid concentrations and the cellular expression of receptors, but also on intracellular enzymes that critically determine to which extent glucocorticoids gain access to receptors in an active forms. 11beta-hydroxysteroid dehydrogenases (11beta-HSD's) catalyze the interconversion of the principal active 11-hydroxy-glucocorticoid (cortisol in man) and their inactive 11-keto metabolites (cortisone in man).
The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) inter-converts inactive into active glucocorticoids, thereby playing a major role in local modulation of cellular agonist concentration and thus activation of corticosteroid receptors in target tissues. In a recent study made by F. Hoffmann-La Roche differences in gene expression in lean and obese man were analyzed using gene array technology in order to identify specific changes in gene expression that might be associated with insulin resistance or altered metabolism. This study revealed that the mRNA for 11beta-HSD1 is approximately two-fold up regulated in adipose tissue in obese individuals. Moreover, overexpressing 11beta-HSD1 in adipocytes of mice led to visceral obesity and to a syndrome-X like phenotype (Masuzaki H. et al., Science. 2001 Dec. 7; 294(5549):2166-70.). Taken together, these data very strongly support an important role of 11beta-HSD1 in the induction of obesity and the impairment of glucose homeostasis and lipid parameters. Thus, selective inhibition of this enzyme could lower blood glucose levels in Type 2 diabetic patients, normalize elevated lipid parameters and/or reduce weight in obese subjects.
The first pharmacological indication that 11beta-HSD1 inhibition in man might have beneficial effects were obtained by using carbenoxolone, an anti-ulcer drug which inhibits both 11beta-HSD1 and the related enzyme 11beta-HSD2. Treatment with carbenoxolone led to an increase in insulin sensitivity indicating that that inhibition of 11beta-HSD1 may reduce cellular cortisol levels and therefore minimizing some of its deleterious effects. (Walker et al. 1995; J. Clin. Endocrinol. Metab. 80, 31155-3159).
11beta-HSD1 is expressed in many tissues including liver, adipose tissue, vascular smooth muscles, pancreas and brain. Its activity is dependent on NADP(H) and it has a relatively low affinity for its substrate (compared to 11beta-HSD2). 11beta-HSD1 in tissue homogenates and when purified is bidirectional, exhibiting both 11beta-dehydrogenase and 11beta-reductase reactions, with greater stability of the dehydrogenase activity (P. M. Stewart and Z. S. Krozowski, Vitam. Horm. 57 (1999), pp. 249-324). However, when the enzyme activity is tested in intact cells, the 11beta-reductase activity predominates, which regenerates active glucocorticoids from inert 11-keto forms. Such glucocorticoid regeneration will increase effective intracellular glucocorticoid levels and thereby amplifying glucocorticoid activity. It is this elevated cellular cortisol concentration that might lead to increased hepatic glucose production, adipocyte differentiation and insulin resistance.
Inhibition of 11beta-HSD1 should not only reduce the typical Syndrome-X/Diabetes associated symptoms, but it should also be safe and without major side effect. Studies with the unspecific inhibitor carbenoxolone highlight the importance of developing specific 11beta-HSD1 inhibitors. The inhibition of the 11beta-HSD2 enzyme is badly tolerated and results in increased blood pressure. In contrast inhibition of 11beta-HSD1 should be well tolerated since 11beta-HSD1 knockout mice were found to be healthy and to resist hyperglycemia provoked by obesity or stress (Kotelevtsev Y. et al., Proc Natl Acad Sci USA. 1997 Dec. 23; 94(26):14924-9). Similar upon starvation these mice had attenuated activation of key hepatic enzymes that are involved in gluconeogenesis. In addition, these mice had improved lipid and lipoprotein profiles suggesting that inhibition of HSD1 might be highly efficacious and safe. Recent reports indicate that 11beta-HSD1 inhibitors might also be beneficial to reduce high blood pressure (Masuzaki H. et al., J Clin Invest. 2003 July; 112(1):83-90; Rauz S. et al., QJM. 2003 July; 96(7):481-90), to improve cognition (Sandeep T C. et al., Proc Natl Acad Sci USA. 2004 Apr. 27; 101 (17):6734-9) or to improve Alzheimer associated deficits. Taken together 11beta-HSD1 inhibition might be a safe and efficacious approach to treat symptoms of diabetes, obesity and other diseases.